Ions tuning water flooding experiments and interpretation by thermodynamics of wettability

Abstract

Abstract In recent years, ions tuning water flooding (ITWF) has become a promising technique to recover oil in sandstone reservoirs. In view of research results for the last decade, it is acknowledged that substantial oil recovery beyond conventional waterflooding from sandstone is due to the wettability alteration. However, the major contributor to wettability alteration is still uncertain. Therefore, this paper investigates this major mechanism and shows how it is involved in the process of IOR. Rock surface chemistry and wettability alteration studies were conducted to explain the mechanisms involved for improving oil recovery in low permeability sandstone reservoirs. Moreover, the thermodynamics of wettability during the ITWF was analyzed to characterize the surface forces between the surfaces of oil/water and water/rock. The major mechanism of ITWF to recover incremental oil was confirmed by coreflood experiments with five different brines. Zeta potential results showed that decreasing divalent cations and salinity makes the electrical charges become strongly negative at both oil/brine and brine/rock interfaces, which result in elevating the repulsive forces between the interface of oil/water and interface of water/rock, and as a result the rock turns more water-wet, which was confirmed by thermodynamics characterization. Different coreflood experiments showed that ITW (0.5569%) with 0.023% divalent cation as secondary (starting at Swi) and tertiary (starting at Sorw) modes recovered 15.6% and 15.1% OOIP, respectively. It demonstrates that the ITW can improve oil recovery in secondary and tertiary modes. Injecting brine (0.2% NaCl) as tertiary mode recovered 10.3% OOIP after 5.8% NaCl solution was flooded at the secondary mode. Furthermore, 10% NaCl as tertiary mode only recovered 3.3% OOIP after the formation brine flooded the secondary mode. It proved that the multi-component ions exchange is not dominant to recover additional recovery without double layer expansion. In conclusion, double layer expansion was caused by the highly negative zeta potential as a result of lower salinity and the divalent cation plays a major role in recovering additional oil. These findings can help in ions tuning waterflood to maintain higher potential to recover oil.